1. Field of the Invention
The present invention relates to a photoelectric conversion device, imaging system, and photoelectric conversion device driving method.
2. Description of the Related Art
Recently, CMOS sensors, which are of an amplification type among photoelectric conversion devices, have widely been used in digital cameras and video cameras because they can provide high image qualities and high resolutions. When the CMOS sensor is irradiated with very strong light as in photographing the sun, charge sometimes leaks from a photodiode to a floating diffusion (to be referred to as an FD) in a pixel irradiated with the strong light. As a result, a signal output to a signal line from an amplification transistor in the pixel irradiated with the strong light in accordance with the FD voltage attenuates abruptly. The level of an image signal (see ΔVOUT shown in FIG. 10) corresponding to this signal drops to black level. This phenomenon is called high-brightness darkening.
To prevent this, Japanese Patent Laid-Open No. 2008-42676 discloses a technique of holding the potential of a signal line, raising the held potential by the threshold voltage of the amplification transistor of a pixel, and then supplying it to the gate of the transistor of a clipping unit 230, as shown in FIG. 1 of Japanese Patent Laid-Open No. 2008-42676. According to Japanese Patent Laid-Open No. 2008-42676, even a small amount of charges leaking to the FD can be detected at high precision to clip the potential of the signal line. This technique is considered to be able to satisfactorily prevent the tone of a pixel corresponding to the signal line potential from dropping to black level.
As the number of pixels of a photoelectric conversion device is increasing, miniaturization of pixels is being promoted. Now, 10-megapixel or more CMOS sensors with a pixel size of 2 μm or smaller are commercially available. At the same time, users request many functions of cameras using CMOS sensors, and demand is particularly high for a high-speed continuous shooting function. To provide it, CMOS sensors need to shorten the readout period for reading out signals from pixels.
However, an increasing number of pixels of a photoelectric conversion device lead to a larger number of pixels connected to a signal line. This makes the signal line longer and its parasitic capacitance larger. As the pixel size becomes smaller, the wiring width of the signal line decreases and its parasitic resistance increases. The time constant becomes large when driving a transistor to raise the signal line potential to a predetermined potential. The time necessary for driving becomes long. In this case, a long time is taken to output a noise signal appropriate for the voltage of an FD (charge-voltage converter) in a pixel to a signal line from an amplification transistor (output unit) while resetting the FD. A period for transferring the appropriate noise signal from the pixel to a CDS circuit (output circuit) also becomes long.